1. Field of the Invention
One embodiment of the present invention relates to a photoelectric conversion element and an imaging device.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, an imaging device, a method for driving any of them, and a method for manufacturing any of them.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A transistor and a semiconductor circuit are embodiments of semiconductor devices. In some cases, a storage device, a display device, an imaging device, or an electronic device includes a semiconductor device.
2. Description of the Related Art
In recent years, consideration is being given to so-called 4K or 8K broadcasting with higher resolution (a larger number of the pixels) for showing an image and a moving image than the resolution of so-called full high definition, which is for the HD television broadcasting, using pixels of 1920×1080 (also referred to as “2K resolution,” “2K1K,” “2K,” and the like). In particular, 8K broadcasting is expected to show images with a sense of reality, a three-dimensional effect, or an immersion effect that have not been able to be experienced by the conventional images; thus, it is highly expected that a revolutionary image experience will be given to the viewers. Note that there are many challenges to be solved such as technology and cost for achieving 4K/8K broadcasting. For example, an imaging device capable of shooting 8K images, a cable which transmits 8K video signals, a tuner included in a display device which receives 8K video signals, a display device capable of displaying 8K images, and the like are needed; however, they are all being developed.
Various devices as described above for the 2K resolution have already been achieved. However, with regard to the resolution of so-called ultra full high definition using pixels arranged in 3840×2160 or in 4096×2160 (also referred to as “4K resolution,” “4K2K,” “4K,” and the like), 4K television broadcasting has not already been provided on a full scale, whereas the display devices began to be sold. Furthermore, with regard to the resolution of so-called super high definition using pixels arranged in 7680×4320 or in 8192×4320 (also referred to as “8K resolution,” “8K4K,” “8K,” and the like), technology development, in any of producing, transmitting, and displaying of the video signals, for reaching the practical use level is awaited. Once 8K television broadcasting is provided, there is an additional possibility of taking 16K and 32K resolution into consideration.
In such a situation, the development of a CMOS sensor having 133,000,000 pixels for a high resolution camera capable of producing 8K images was reported (see Non-Patent Document 1). In order to fabricate such a high resolution sensor, a highly integrated array of pixels is required. High integration of pixels needs reduction in area per pixel.
In the case where the area of a pixel is reduced, the light-receiving area of a photoelectric conversion element included in the pixel also needs to be reduced. When the light-receiving area of the photoelectric conversion element is reduced, it might be difficult to perform imaging under a low illuminance condition because of the decrease in sensitivity to light.
In order to solve such a problem, a photoelectric conversion element utilizing avalanche charge multiplication can be effectively used. However, such a photoelectric conversion element has a relatively large dark current, which might cause deterioration in imaging quality. As a countermeasure, an image sensor in which a dark current can be reduced has been disclosed (see Patent Document 1).